Single and Mixed Oxide-Supported Nickel Catalysts for the Catalytic

878

Energy & Fuels 2008, 22, 878–891

Single and Mixed Oxide-Supported Nickel Catalysts for the Catalytic Partial Oxidation Reforming of Gasoline Hussam H. Ibrahim and Raphael O. Idem* Hydrogen Production Research Group, Process Systems Engineering, Faculty of Engineering, UniVersity of Regina, 3737 Wascana Parkway, Regina, SK, Canada S4S 0A2 ReceiVed October 5, 2007. ReVised Manuscript ReceiVed December 19, 2007

The catalytic partial oxidation of iso-octane and sulfur-free gasoline was studied over Ni-based catalysts supported on CeO2-, La2O3-, ZrO2-, Al2O3-, and their combinations prepared by precipitation and surfactant assisted methods. A comparison of their performance was also made with a Ni/Al2O3 catalyst for the oxidation of pure gasoline surrogate, that is, iso-octane. These catalysts were characterized by specific surface area, temperature programmed reduction (TPR), H2 chemisorption, and X-ray diffraction, and their catalytic performance was tested in a fixed bed tubular reactor. The characteristics of the catalysts were then correlated with their performance. The use of CeO2 as a support prepared by a surfactant approach was found to produce the most stable catalyst for the partial oxidation of both iso-octane and gasoline. TPR results showed that the stability of CeO2 is derived from its enhanced reducibility at lower temperatures as compared to either similar support prepared by other methods or different support prepared by the same method or other methods. Brunauer-Emmett-Teller surface area measurements indicated a 100% enhancement in the CeO2 support specific surface area when the surfactant method was used during preparation as compared with the precipitation method. On the basis of a stability test, 5% Ni CeO2 was found to be the most stable catalyst.

1. Introduction The world has largely depended on energy derived from the combustion of fossil fuels for the major reasons that fossil fuels are abundant and provide a more reliable source of energy. However, the extensive use of these fossil fuels has a drastic impact on our environment in terms of emissions. The question then is how to use this source of energy more responsibly. One alternative is that fossil fuels could be transformed into a more environmentally benign commodity such as hydrogen, which is an energy vector that could be used directly as a fuel in internal combustion engines or indirectly to provide energy in the form of electricity using fuel cells.1 However, the use of hydrogen as an alternative fuel is limited partly because of the unavailability of a well established infrastructure for production, storage, and distribution. This concern could be circumvented if we are able to extract hydrogen at the point of use from a more readily available source with a well established infrastructure. Liquid hydrocarbons in general, and gasoline in particular, are well suited for the production of hydrogen owing to their high energy density, low price, abundance, high H2 storage capacity, and well developed infrastructure network.2 The conversion of gasoline to hydrogen in the presence of a catalyst could be achieved by two main reaction processes, namely, steam reforming (SR) and partial oxidation (POX). SR of gasoline is a highly endothermic process known to have high efficiencies and high H2 yields. However, it consumes a large portion of the fuel to maintain the process temperature. It is also a relatively slow process. On the other hand, POX is highly * To whom correspondence should be addressed. Telephone: +1 306 585 4470. Fax: +1 306 585 4855. E-mail: [email protected]. (1) Navarro, R. M.; Álvarez-Galva’n, M. C.; Rosa, F.; Fierro, J. L. G. Appl. Catal., A 2006, 297, 60–72. (2) Moon, D. J.; Sreekumar, K.; Lee, S. D.; Lee, B. G.; Kim, H. S. Appl. Catal., A 2001, 215, 1–9.

exothermic and sustains heat once initiated. Also, the POX process achieves high conversions in smaller reforming units, has low heat requirements, and does not require steam generation. Hydrogen production by the catalytic POX (CPOX) of liquid hydrocarbon continues to be an active area of research3 even though the reforming of higher hydrocarbon molecules such as gasoline has proven to be challenging. It requires the development of a stable catalyst for the process. It also requires detailed thermodynamic and kinetic knowledge about the reaction.4 In addition, H2 production by the POX reforming of liquid hydrocarbons will produce other carbon-containing products such as CH4, CO2, and CO. Another major challenge is the rapid catalyst deactivation as a result of thermal sintering and coke formation. Coke formation in particular becomes important with heavier hydrocarbon feedstocks such as gasoline as well as the high temperature usually associated with POX reforming to produce H2. Four reactions have been reported to be involved in the process5,6 C8H18 f 8C + 9H2 CH4 a C + 2H2 2CO a C + CO2 CO + H2 a C + H2O

∆H0298 ) 224 kJ mol-1 ∆H0298 ) 75

(1)

-1

kJ mol

(2)

∆H0298 ) -172 kJ mol-1 ∆H0298 ) -131

(3) -1

kJ mol

(4)

Eqs 1 and 2 represent the hydrocarbon dissociation on the metal surface to produce carbon deposits which are either (3) Tadd, A. R.; Chen, X.; Schwank, J. The effects of nickel loading on a Ni/CeO.75ZrO.25O2 autothermal reforming catalyst. Presented at the 2005 AIChe Annual Meeting, Cincinnati, OH, 2005. (4) Bobrova, L.; Korotkich, V.; Sadykov, V.; Parmon, V. Chem. Eng. J. 134, (1–3), 145–152. (5) Villegas, L.; Guilhaume, N.; Provendier, H.; Daniel, C.; Masset, F.; Mirodatos, C. Appl. Catal., A 2005, 281, 75–83. (6) Trimm, D. L. Catal. Today 1997, 37, 233–238.

10.1021/ef7005904 CCC: $40.75  2008 American Chemical Society Published on Web 03/04/2008

Single and Mixed Oxide-Supported Nickel Catalysts

gasified or dissolved in the active metal to produce coke.6 On the other hand, eqs 3 and 4 depict the formation of carbon via the CO disproportionation and CO hydrogenation reactions, respectively. It has been suggested7–10 that carbon formation on metal surfaces generated during the reaction could take more than one form depending on the reaction temperature. Amorphous coke film predominates in the lower temperature range of 673–873 K7,8 whereas graphitized (whisker-type) carbon predominates at temperatures higher than or equal to 973 K.9,10 It has also been reported11–13 that the diffusion and segregation of carbon are dependent on the metal surface structure. For example, the carbon on Ni(110) can diffuse more readily into the bulk than that on Ni(100), and consequently carbon formation is more difficult on Ni(111).11 Also, carbon whiskers form preferentially on large Ni particles because the critical nucleus of grapheme is large and nucleation into whiskers cannot proceed if the facets or step edges of the Ni particles are too small.12 Furthermore, the carbon adsorbed on smaller metal particles diffuses with greater difficulty than that on the larger particles.13 This means that structure sensitivity of carbon formation can provide the possibility for inhibition of carbon deposition by modification of the catalyst surface structure. Trimm6 studied the formation of coke during the reforming of hydrocarbons and concluded that among the strategies for coke minimization would be the detailed studies of catalyst supports. Exploring the supports’ chemistry and the advantages of adding heavier rare earth oxides to Ni-based catalysts could enhance coke gasification, and this appears to offer the best chances of improved control of coking during reforming reactions.6 Catalyst preparation and the amount and nature of active metal dispersed on an oxide support material are believed to also play a major role in the activity, selectivity, and stability of a catalyst.14–17 The majority of catalysts investigated in the literature for the POX of gasoline or gasoline surrogates contain one or more of the platinum group metals. While noble metalcontaining catalysts are proven to have good sulfur tolerance,5,18,19 deactivation because of carbon deposition can also be observed.5,20 Also, the high cost of noble metals (even at low loadings such as 0.1 wt % on the catalyst), limited reserves, and lack of feasibility for practical use limit their application.21–23 On the other hand, nickel-based catalysts, owing to their comparable reforming activity and high CsC bond-breaking activity, availability, and low cost, are a more appealing and practical (7) Duprez, D.; Fadili, K.; Barbier, J. Ind. Eng. Chem. Res. 1997, 36, 3180–3187. (8) Verykios, X. E. Int. J. Hydrogen Energy 2003, 28, 1045–1063. (9) Park, C.; Keane, M. A. J. Catal. 2004, 221, 386–399. (10) Bartholomew, C. H. Catal. ReV. - Sci. Eng. 1982, 24, 67–112. (11) Bradford, M. C. J.; Vannice, M. A. Catal. ReV. - Sci. Eng. 1999, 41, 1–42. (12) Bengaard, H. S.; Norskov, J. K.; Sehested, J.; Clausen, B. S.; Nielsen, L. P.; Molenbroek, A. M.; Rostrup-Nielsen, J. R. J. Catal. 2002, 209, 365–384. (13) Hu, Y. H.; Ruckenstein, E. AdV. Catal. 2004, 48, 297–345. (14) Hohn, K. L.; Schmidt, L. D.; Reyes, S. C.; Feeley, J. S. U.S. Patent Application Publication, US 2001/0027258, 2001. (15) Hutchings, G. J. Catal. Lett. 2001, 75, 1–12. (16) Pinna, F. Catal. Today 1998, 41, 129–137. (17) Perego, C.; Villa, P. Catal. Today 1997, 34, 281–305. (18) Peppley, B. A. Presented at the 18th Canadian Symposium on Catalysis, Montreal, Canada, May 16–19, 2004. (19) Becerra, A. M.; Iriarte, M. E.; Castro-Luna, A. D. React. Kinet. Catal. Lett. 2003, 79 (1), 119–125. (20) Puolakka, K. J.; Juutilainen, S.; Krause, A. O. I. Catal. Today 2006, 115, 217–221. (21) Qia, A.; Wang, S.; Fu, G.; Ni, C.; Wu, D. Appl. Catal., A 2005, 281, 233–246. (22) Xu, S.; Zhao, R.; Wang, X. Fuel Process. Technol. 2004, 86 (2), 123–133. (23) Wang, S.; Lu, G. Q. M. Appl. Catal., B 1998, 16 (3), 269–277.

Energy & Fuels, Vol. 22, No. 2, 2008 879

alternative to noble metals from the industrial standpoint.24 The development of robust nickel-based catalysts that are resistant to carbon deposition and that will exhibit stable operation for an extended period of time is urgently needed. In this regard, innovative support materials,20,23,25 improved Ni dispersion,24–26 introduction of alkalinity,27,28 and use of promoters29–31 have been applied to lower or eliminate the carbon deposition. On the basis of studies for dry reforming of methane reaction,32 the nature of the support can influence the catalytic performance of supported Ni catalysts for POX of gasoline because it can affect metal dispersion and thermal stability. Cerium, lanthanum, zirconium, and their combinations have drawn much attention recently as materials that could improve textural properties, enhance activity, improve active metal dispersion, eliminate or reduce coke formation, and prevent thermalsinteringwhenusedasasupportmaterialorpromoter.1,29,33–40 The role of Ce addition is to enhance the acidity and the hydrothermal stability of a catalyst. Acid sites of catalysts promote the oxidation reforming reaction by promoting the cracking of CsC bonds of the liquid hydrocarbon feed.41 Ceria also possesses a dual-function mechanism in which CeO2 transfers oxygen to the supported metal and is, in turn, reoxidized by O2 during POX reforming. Moreover, CeO2 is proven to enhance the catalytic activity, selectivity, and thermal stability of catalysts.38 On the other hand, La2O3 enhances steam adsorption which facilitates the gasification of the surface carbon.42 Also, the enhanced stability on the Ni/La2O3 catalyst toward POX of methane to synthesis gas is attributed to the isolation of nickel ensembles by LaOx species.25 Coke deposition also decreases when Ni is impregnated on supports that present a marked Lewis alkalinity/acidity and improved thermal stability such as ZrO2 and La2O3.8,10,13 Similarly, strong metal–support interaction between Pt and Zr is shown to produce a good reforming catalyst.43 Another report concluded44 that Ni/ZrO2 catalytic stability depends greatly on the preparation method and the support precursor. (24) Ferrandon, M.; Krause, T. Appl. Catal., A 2006, 311, 135–145. (25) Horiuchi, T.; Sakuma, K.; Fukui, T.; Kubo, Y.; Osaki, T.; Mori, T. Appl. Catal. 1996, 144, 111–120. (26) Bradford, M. C. J.; Vannice, M. A. Appl. Catal., A 1996, 142, 73– 96. (27) Trovarelli, A. Catalysis by Ceria and Related Materials; Imperial College Press: Universita di Udine, 2002; p 2. (28) Laosiripojana, N.; Assabumrungrat, S. Appl. Catal., B 2005, 60, 107–116. (29) Zhang, J.; Xu, H. a.; Jin, X.; Ge, Q.; Li, W. Appl. Catal., A 2005, 290, 87–96. (30) Lemondiou, A. A.; Goula, M. A.; Vasalos, I. A. Catal. Today 1998, 46, 175–183. (31) Aparicio, L. A. J. Catal. 1997, 165, 262–274. (32) Kumar, P.; Sun, Y.; Idem, R. Energy Fuels 2007, 21 (6), 3113– 3123. (33) Pengpanich, S; Meeyoo, V.; Rirksomboon, T.; Schwank, J. Appl. Catal., A 2006, 302, 133–139. (34) Sánchez-Sánchez, M. C.; Navarro, R. M.; Fierro, J. L. G. Int. J. Hydrogen Energy 2007, 32, 1462–1471. (35) Rajesh, B. B.; Mizuno, A.; Ichikawa, M. Catal. Lett. 2005, 276, 169–177. (36) Hou, Y.; Wang, Y.; Hea, F.; Mi, W.; Li, Z.; Mi, Z.; Wu, W.; Min, E. Appl. Catal., A 2004, 259, 35–40. (37) Radwan, N. R. E.; Fagal, G. A.; El-Shobaky, G. A. Colloids Surf., A 2001 (a) , 178, 277–286. (38) Radwan, N. R. E. Appl. Catal., A 2004 (b) , 257, 177–191. (39) Ferrandon, M.; Björnbom, E. J. Catal. 2001, 200, 148–159. (40) Stagg-Williams, S. M.; Noronha, F. B.; Fendley, G.; Resasco, D. E. J. Catal. 2000, 194, 240–249. (41) Wang, L.; Murata, K.; Inaba, M. J. Power Sources 2005, 145, 707– 711. (42) Garcia, L.; French, R.; Czernik, S.; Chornet, E. Appl. Catal., A 2000, 201, 225–239. (43) Lercher, J. A.; Bitter, J. H.; Hally, W.; Niessen, W.; Seshan, K. Stud. Surf. Sci. Catal. 1996, 101, 463–472.

880 Energy & Fuels, Vol. 22, No. 2, 2008

The catalyst prepared by impregnation of ultrafine Zr(OH)4 particles with nickel nitrate showed high and stable activity for methane dry reforming. Also, ZrO2 incorporation in the support has been reported to provide high thermal and mechanical stability thereby enabling the possibility to work at relatively higher temperatures while minimizing coke formation on the catalyst surface.43,45 The addition of ZrO2 to CeO2 also leads to improvements in oxygen storage capacity (OSC), thermal stability, and improved metal dispersion.27,46,47 Furthermore, it has been observed that surface reduction of ceria is greatly enhanced when ZrO2 is incorporated into CeO2 or when the two form a solid solution.27 Alumina support modified with lanthanum and cerium was found to enhance the catalyst activity during the autothermal reforming (ATR) of hexadecane.1 The presence of La2O3 in the support formulation increases the number of Ni-Ce and Ni-Al surface interactions which play a key role in both lowering the carbon deposition on the catalysts’ surface and increasing the thermal stability of Ni during the hexadecane ATR. A few studies on the role of adding rare earth oxides such as CeO2 and mixed oxide supports such as CeO2-ZrO2 in enhancing the performance of Ni-based catalysts have been reported. For example, Wang et al.41 studied the role of Ce addition to ZSM-5-supported Ni-based catalyst in the POX reforming of a mixture of hydrocarbon fuel as a model gasoline. However, the model gasoline contained only two components, namely, 75% methyl cyclohexane and 35% toluene. This may not be an accurate reflection of the major constituents of gasoline. Also, results were not presented in favor of showing that Ce containing catalyst was stable for the POX reforming reaction. Kang et al. in two separate studies48,49 reported the ATR of gasoline and diesel over a Pt-Ce-based proprietary catalyst. Both studies focused on the process thermodynamics and reformability of the main constituents of diesel and gasoline, but limited studies have been done in terms of catalyst characteristics related to their performance. Another report50 addressed the application of a two stage reformer to minimize coking during gasoline ATR. The first stage used 50% Ni/La2O3 and the second used 8% Ni/La2O3 both supported on commercial MgO-R-Al2O3. The authors observed deactivation of the catalysts by coke formation in the absence of a prereformer unit. Navarro et al.1 studied the influence of different promoters (Ce and La) in the H2 yield during ATR of hexadecane on Ni and Pt Al2O3-supported catalysts. These authors considered that the high thermal stability and coke resistance of the Ni-based catalyst stemmed from the enhanced metal–support interaction due to the addition of Ce and/or La to the Al2O3 support. Perhaps the most relevant report in the open literature is the recent work of Pengpanich et al.33 in which the authors investigated the stability of Ni/Ce0.75Zr0.25O2, prepared by conventional impregnation, for iso-octane POX in the temperature range of 400–800 °C. The conversion of iso-octane was complete, and the catalyst was found to be very stable over 6 h time-on-stream (TOS). However, the authors did not give a justification for the superior behavior of the Ni/Ce0.75Zr0.25O2 system. One of the reasons (44) Wei, J.-M.; Xu, B.-Q.; Li, J.-L.; Cheng, Z.-X.; Zhu, Q.-M. Appl. Catal., A 2000, 196, L167–L172. (45) Yamaguchi, T. Catal. Today 1994, 20, 199–218. (46) Kaspar, J.; Fornasiero, P.; Graziani, M. Catal. Today 1999, 50, 285–298. (47) Pengpanich, S.; Meeyoo, V.; Rirksomboon, T.; Bunyakiat, K. Appl. Catal., A 2002, 234, 221–233. (48) Kang, I.; Bae, J. J. Power Sources 2006(a) , 159, 1283–1290. (49) Kang, I.; Bae, J.; Bae, G. J. Power Sources 2006. (b) , 163, 538– 546. (50) Chen, Y.; Xu, H.; Jin, X.; Xiong, G. Catal. Today 2006, 116, 334– 340.

Ibrahim and Idem

for having 100% conversion, regardless of the catalyst used, could be the very dilute nature of the iso-octane feed. This was evident when the authors tested a more conventional catalyst (Ni/Al2O3), which gave almost identical results under the same conditions. The objective of the present paper is to study the influence of the support material and its preparation technique on the catalytic activity, as well as the chemical and thermal stability of Ni-based catalysts for CPOX of gasoline. More specifically, we have tried to develop a system of supports that would offer a better control of coking during POX reforming. The supports were intended to prevent formation of large Ni particles (i.e., ensembles) thereby preventing the formation of carbon whiskers eventually leading to a minimization of catalyst deactivation. Consequently, single oxide supports, namely, CeO2, ZrO2, La2O3, Al2O3, and their combinations, have been explored for their ability to provide such support for nickel. In particular, a surfactant assisted (SA) approach of support preparation technique, which is believed to lead to the reduction of particle size of the support material to such an extent that it becomes comparable to the metal particles resulting in a nanocomposite of supported metal catalyst, was one of the support preparation methods adopted. The use of the SA method of catalyst preparation for reforming reactions is not quite new. However, the effects of the support material and their preparation technique on the catalytic activity, as well as the chemical and thermal stability of Ni-based catalysts for CPOX of gasoline to produce hydrogen, have not been reported before in the open literature. This was compared to a more conventional deposition/precipitation method of support preparation. The catalytic activity and stability were correlated to the support choice and preparation method for the POX of gasoline. These results are presented and discussed in this paper. 2. Experimental Section 2.1. Chemicals. Ultrahigh pure hydrogen (5%) balanced in nitrogen, hydrocarbon-free air (99.999%) used as a source for O2, and nitrogen (99.999%) were used for catalyst pretreatment and during the POX reaction. Helium (99.999%) was used as a carrier gas for the gas chromatography (GC) system. Nickel nitrate (Ni(NO3)2 · 6H2O), zirconium nitrate (ZrO(NO3)2 · 7H2O), cerium nitrate (Ce(NO3)2 · 6H2O), lanthanum nitrate (La(NO3)3 · 6H2O), aluminum nitrate (Al(NO3)3 · 9H2O), ammonium hydroxide (NH4OH), and cetyltrimethylammonium bromide (CTAB, C16) were obtained from Aldrich Co. 2.2. Catalyst Preparation. A series of Al2O3, CeO2, ZrO2, La2O3, and their combinations supported Ni catalysts were prepared using two methods of preparation, namely, the SA and the precipitation (PT) methods. In all the prepared catalysts the nickel concentration was 5 wt %. The 5% nickel loading was selected on the basis of two extensive previous studies.32,57 Results from both studies show that lower nickel loadings, typically 3–5 wt %, exhibited better degree of dispersion and high level of stable conversion and selectivity. 2.2.1. Precipitation Method. Predetermined concentration(s) of the intended oxide or mixed oxides support material salt(s) were first dissolved in a measured quantity of deionized water and mixed for 0.5 h. Aqueous ammonia was added slowly to the solution under continuous stirring until a pH of 11 was attained. The solution was stirred overnight and then washed thoroughly with hot water. The wet catalyst cake was dried overnight at 90 °C and then calcined at 650 °C for 5 h. Nickel was added to the support by means of nitrate salt deposition. The idea here was to hydrolyze the salt solution for nickel in a controlled environment. A certain amount of Ni(NO3)2 · 6H2O was dissolved in deionized water before adding it into a container that had the appropriate amount of oxide or mixed oxide support slurry. The nickel nitrate salt was brought into



Figure 1. Schematic of the synthetic gasoline POX reforming experimental setup.

precipitation by slowly adding aqueous ammonia till a pH of 11 was reached, and the solution was maintained under stirring for 24 h. The resulting catalyst slurry was filtered, washed thoroughly with warm water, dried overnight at 90 °C, and calcined at 650 °C for 5 h. 2.2.2. Surfactant Assisted Method. This method of catalyst development utilizes the interaction of hydrous oxide of the support material, that is, CeO2, ZrO2, and La2O3, with the cationic surfactant under basic condition followed by nickel impregnation. The method was the subject of a recent patent.51 An aqueous solution of the surfactant CTAB (0.1 M, Aldrich) was prepared and mixed with another aqueous solution containing the stoichiometric quantities of the designated oxide or mixed oxide support (molar ratios of pure oxide support/CTAB and mixed oxide support/CTAB of 1.2 and 0.8, respectively). Deionized water was used as the solvent for the precursor materials. The mixture was stirred for 1 h. An aqueous solution of ammonia (28-30%) was added dropwise, under vigorous stirring, to the above solution over a period of 90 min. The initial pH before adding the ammonia solution was less than 5 for all the prepared supports at 34 °C, which increased to 11 upon completion. This caused the precipitation of hydrous oxide to a gelatinous solid. The slurry was stirred for 2 h in a glass vessel and then sealed and placed in an oven maintained at 90 °C for 5 days. After aging, the mixture was cooled to room temperature and the precipitate filtered and washed with abundant amounts (4 L) of hot water to remove the free surfactant not incorporated within the oxide. The resulting wet cake was dried at 90 °C overnight and calcined at 650 °C for 5 h under airflow to remove the surfactant. The incorporation of nickel was achieved by means of wet impregnation of aqueous solution of Ni(NO3)2 · 6H2O. A premea(51) Idem, R. O.; Kumar, P.; Sun, Y. U.S. Patent Application Publication, US 20060216227, 2006.

sured quantity of Ni(NO3)2 · 6H2O was dissolved in deionized water before adding it into a container that had the appropriate amount of oxide or mixed oxide support slurry. The final slurry was left overnight under stirring to ensure enough contact time. After impregnation, the samples were degassed in a vacuum controlled rotary evaporator to slowly remove the water and to let the metal salt solution fully fill the pores of the support. The resulting catalyst was dried overnight at 110 °C and then calcined at 650 °C for 5 h. 2.3. Characterization of Catalysts. Elemental analyses of the fresh catalysts were performed using the inductively coupled plasma-mass spectroscopy (ICP-MS, Varian Inc.) technique. Samples were first dissolved in a mixture of acidic solutions of HCl, HNO3, and H2SO4, microwaved for 30 min, and diluted to concentrations within the calibration range of the instrument. The surface areas (Brunauer-Emmett-Teller, BET, method) of fresh and spent catalysts were determined by nitrogen adsorption by using the nitrogen adsorption–desorption isotherms obtained at the temperature of liquid nitrogen (77 K) using a surface area instrument (Micromeritics ASAP 2010). Prior to analysis, each sample was outgassed at 523 K under vacuum for at least 10 h. The degree of nickel dispersion was determined by H2 chemisorption (Micromeritics ASAP 2010). Approximately 0.7 g of fresh catalyst was placed in a quartz sample cell. The sample was oxidized and evacuated at 623 K and then reduced in flowing H2 at 973 K prior to analysis. The sample was then cooled to the analysis temperature of 308 K by purging with helium. The method of the double isotherm was used to determine the irreversibly bound chemisorbed H2, determined by extrapolating the linear part of the isotherm to zero pressure which should correspond to H2 adsorbed on the metal surface. The nickel dispersion was calculated by assuming the adsorption stoichiometry of one H2 atom per nickel surface atom.


Ibrahim and Idem

Table 1. Physical Properties of Fuels and Their Percentage Composition in Synthetic Gasoline component

density (g mL-1)

molecular weight (g mol-1)

purity (%)

vol (%)

wt (%)

2,2,4-trimethylpentane methylcyclohexane 1,2,4-trimethylbenzene hexane 1-octene

0.69 0.77 0.88 0.66 0.72

114.2 98.2 120.2 86.2 112.2

99.9 99.5 99.0 99.9 99.0

50.0 5.0 35.0 5.0 5.0

45.5 5.1 40.4 4.3 4.7

Table 2. Catalysts Chemical Composition (ICP-MS) of Calcined Catalysts Nia

a

(wt %)

Nib (wt %)

Alb (wt %)

Ceb (wt %)

Zrb (wt %)

Lab (wt %)

Ni/Al Ni/Ce Ni/La Ni/Zr Ni/Ce0.6Zr0.4 Ni/La0.6Zr0.4 Ni/La0.6Al0.4 Ni/Ce0.6La0.4

5 5 5 5 5 5 5 5

4.2 4.5 5.4 5.6 5.2 6.1 5.0 6.7

PT Method 95.8 c c c c c 11.9 c

c 95.5 c c 72.0 c c 44.4

c c c 94.6 22.8 29.4 c c

c c 94.6 c c 64.5 83.1 48.9

Ni/Ce Ni/La Ni/ZrO2 Ni/Ce0.6Zr0.4 Ni/La0.6Zr0.4 Ni/La0.6Al0.4 Ni/Ce0.6La0.4

5 5 5 5 5 5 5

5.6 6.0 5.8 5.1 4.7 5.3 5.2

SA Method c c c c c 12.2 c

94.4 c c 75.9 c c 45.4

c c 94.2 19.0 31.2 c c

c 94.0 c c 64.1 82.5 50.6

Nominal values. b ICP-MS determined values. c Not detected.

Catalyst crystalline phase recognition and a possible measure of Ni crystallite size were determined by powder X-ray diffraction (XRD) using a D8/GADDS diffractometer (Bruker, AXS). The XRD apparatus used the following parameters: nickel-filtered Cu KR radiation of 40 mA, 40 kV, λ ) 0.15418 nm, Bragg angle 2θ scanning range of 10-80°, and a scan step size of 0.04°. Temperature programmed reduction (TPR) studies were performed in a fixed bed quartz tube (model TPD/TPR ChemBET3000, Quantachrome) equipped with a TCD detector. The TPR was conducted by heating approximately 100 mg of catalyst from room temperature to 1273 K at a rate of 15 K min-1 in a gas mixture of 5% H2 in N2 at a flow rate of 35 mL min-1. Prior to analysis, the samples were purged with N2 at 250 °C to remove any moisture, adsorbed gases, and other contaminants. The samples were then cooled to room temperature under the same N2 blanket. 2.4. Experimental Setup and Reactor System. The schematic diagram for the experimental setup used in the POX reforming of synthetic gasoline is shown in Figure 1. The reforming reactor used for data collection was a fixed-bed flow reactor made of an Inconel 600 tube (∅ 0.0127 m i.d. and 0.457 m length) encased in an electric furnace. The reactor was well insulated to minimize heat losses for good temperature control. The reaction temperature was controlled by a PID temperature controller connected to the furnace whereas the process temperature was monitored by means of a sliding thermocouple placed at the center of the catalyst bed. The error in temperature measurement was within (1 K. The catalyst to diluent ratio was established to avoid preferential gas flow paths and hot spots. To approach plug flow conditions and minimize back mixing and channelling, certain operating criteria as prescribed by Froment and Bischoff were used.52 Results from our earlier study53 revealed that intraparticle pore diffusion limitations are negligible for the range of particle sizes used for this study.53 To avoid wall effects on the reactor, the ratio of catalyst bed diameter (Db) to the catalyst particle size (Dp), that is, Db/Dp, was greater than 10. Also the ratio of the catalyst bed length (Lb) to the catalyst particle size, that is, Lb/Dp, was greater than 50 so as to approach plug-flow conditions. (52) Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design, 2nd ed.; Wiley: New York, 1990. (53) Ibrahim, H.; Idem, R. Chem. Eng. Sci. 2007, 62 (23), 6582–6594.

2.5. Catalytic Activity Measurements. The catalytic activity of the prepared catalysts was tested for the POX reforming of gasoline and iso-octane using the POX-reforming system described above. Typically, 0.2 g of catalyst having particle size of 780 × 10-6 m (i.e., passing through ASTM mesh No. 20 and holding on mesh No. 25) was mixed with R-Al2O3, that is, inert material, of the same particle size and is loaded in the middle of the reactor, supported on quartz wool. The catalyst activation prior to any catalytic activity test was carried out in situ by treatment with a 5% H2 balanced in N2 flow of 100 mL min-1 at 973 K for 2 h. The 973 K was reasonably high enough to reduce the relevant nickel species but without deactivating the catalysts. Reducing at very high temperatures (>800) will most likely introduce sintering and cause nickel particle agglomeration, which will eventually contribute to the catalyst rapid deactivation. The pretreatment gases were flushed from the reactor with N2 prior to the introduction of reactants. All runs were conducted at atmospheric pressure in the temperature range of 673–1023 K, feed molar ratio (O/C) range of 0.2–1.0 (i.e., air flow rate 390 mL/min, hydrocarbon flow rate 0.14–0.56 mL/min), and at a gas hourly space velocity (GHSV, defined as the ratio of the gas volumetric flow rate in mL/h to the weight of catalyst in grams at STP) of 1.6 × 105 to 2.8 × 106 h-1. Pressures above 3 psig were not encountered. The gases were regulated and delivered via a digital mass flow controller (DFC26S, Aalborg); the liquid hydrocarbon flow rate was measured and controlled by a syringe pump (KDS200, KD Scientific Inc.). After reaching steady state at the designated temperature, samples were taken at intervals of 45 min. The closed mass balances for all concerned elements, that is, carbon, oxygen, and hydrogen, and the absence of significant amounts of other carbon-containing products allow us to discuss the reaction on the basis of a carbon monoxide, carbon dioxide, and methane formation. Blank runs in the absence of catalyst were also conducted to determine the extent of gas phase reactions. The results were evaluated in terms of H2 yield (the molar ratio of H2 in the product to that in the reactant gasoline or iso-octane) and hydrocarbon or carbon conversion (the molar ratio of carbon in C1 (CO, CO2, and CH4) species in the products to that of total carbon in the feed). All the experimental conditions (i.e., reduction temperature, reaction temperature, feed ratio, and flow rates) were maintained the same for comparison of



Table 3. BET Surface Area, Pore Diameter, and Pore Volume for Catalysts BET surface areas (m2 g-1)

reduction in surface area (%)

pore diameter (nm)

pore volume (cm3 g-1)

F

AI

AR

AI

AR

F

AI

F

AI

Al2O3 CeO2 La2O3 ZrO2 Ce0.6Zr0.4 Ce0.6La0.4 La0.6Zr0.4 La0.6Al0.4

185.2a 65.0a 51.4a 112.8a 78.0a 57.7a 86.2a 114.0a

161.2b 41.6b 15.7b 88.6b 27.0b 29.6b 51.3b 83.2b

29.5c 14.3c 3.2c 11.0c 17.1c 14.3c 26.1c 53.2c

13.0 36.0 69.5 21.5 65.4 48.7 40.5 27.0

(a) Prepared by PT 81.7 65.6 79.6 87.6 36.7 51.7 49.1 36.1

10.9 5.8 11.3 10.4 9.4 10 7.5 9.8

11.7 6.3 13.4 11.3 10.2 12.2 8.7 10.4

0.47 0.10 0.08 0.26 0.17 0.14 0.15 0.42

0.45 0.08 0.07 0.20 0.12 0.07 0.08 0.40

CeO2 La2O3 ZrO2 Ce0.6Zr0.4 Ce0.6La0.4 La0.6Zr0.4 La0.6Al0.4

155 60.4 124 149.5 98.7 156.3 46.9

135.2 40.9 80.4 136.1 67.3 80.2 33.9

95.2 19.4 18.9 85.4 22.1 27.7 16.4

12.8 32.3 35.2 9.0 31.8 48.7 27.7

(b) Prepared by SA 29.6 52.6 76.5 37.3 67.2 65.5 51.6

3.3 5.3 8.7 4.2 10.7 6.0 7.0

3.5 6.0 9.8 4.5 14.4 8.0 7.3

0.23 0.16 0.30 0.23 0.34 0.30 0.10

0.21 0.13 0.22 0.19 0.29 0.23 0.07

a

Fresh calcined support or catalyst. b Catalyst after 5 wt % Ni impregnation. c Catalyst after reaction (h TOS at 700 °C).

the catalysts. Also the catalyst calcination temperature, catalyst weight, and catalyst to diluent ratio were kept the same. The only variable was the type of catalyst. Experimental runs were reproducible with an error of about 5%. 2.5.1. Feed Composition. The main reactants for this process were synthetic gasoline and oxygen. Synthetic gasoline is henceforth defined as the combination of normal paraffins (P), iso-paraffins (I), olefins (O), naphthenes (N), and aromatics (A) with purity 99%+ (Aldrich Co.). The physical properties of fuels as well as their percentage contribution in the making of synthetic gasoline are given in Table 1. The average molecular formula, based on the weighted average of these components, was C8.27H15.10, and the average molecular weight was 114.51 g mol-1. 2.5.2. Product Analysis. The reaction products in the reformate were first passed through a water-cooled condenser and then through a knock out ice trap to separate permanent gases from the condensate. The condensate from the reactor was considered to be unconverted/reformulated hydrocarbon. The composition of the gas effluent was monitored by an online GC (HP 6890, Agilent Technologies) using a molecular sieve and Haysep columns (Alltech Associates) equipped with a thermal conductivity detector (TCD) with helium as the carrier gas. The carbon material balance showed about 5–10 wt % loss, which was attributed to coke deposition on the catalytic surface, unconverted hydrocarbon condensate, and/or analytical error.

3. Results and Discussion 3.1. Support and Catalyst Characterization. 3.1.1. Elemental Composition (ICP-MS). The elemental composition, expressed as weight percentages, of the calcined catalysts are summarized in Table 2. A group of eight catalysts was chosen for each preparation method. Catalysts for which the support was prepared by the precipitation and the SA method are categorized as PT and SA, respectively. The results show that, in general, the weight loading determined by ICP-MS is in agreement with the target (nominal) value for metal loading, indicating the effectiveness of the catalyst preparation procedure. 3.1.2. BET Surface Area, Pore Diameter, and Pore Volume. The textural properties and porosity (specific surface area, pore diameter, and pore volume) of fresh supports and catalysts and spent catalysts are presented in Table 3a for catalysts prepared by PT and Table 3b for catalysts prepared by the SA method. The results show that the introduction of nickel into the texture of single oxide supports was accompanied by a drop in surface area and pore volume and an increase in the pore size regardless of the preparation method, as expected.

This could be attributed to the deposition of nickel particles on the pore structure of supports. The BET analysis for single oxide supports after the reforming reaction also showed a drop in surface area and pore volume for supports prepared by both methods. Cheekatamarla and Finnerty54 indicated that such a loss in surface area could be attributed to the thermal and mechanical instability of catalyst and carbon deposition during the reforming reaction. However, single oxide supports prepared by the SA method maintained a relatively higher percentage of the original surface area both after nickel impregnation and after reaction compared to the ones prepared by PT. It is worthy to note that La2O3 prepared by PT suffered a large reduction in surface area after nickel incorporation which could indicate that 5% nickel may not be the optimum concentration for La2O3 catalysts. CeO2 single oxide support prepared by the SA method maintained about the highest surface area after nickel impregnation (13% decrease) and after the reforming reaction (30% decrease). The results shown in Table 3a,b show that the specific surface area and pore volume for single oxide supports increased in the following order: La2O3(PT) < La2O3(SA) < CeO2(PT) < ZrO2(PT) < ZrO2(SA) < CeO2(SA) < Al2O3(PT). The results from Table 3a,b for single oxide supports indicate that the SA method of preparation provided a large specific surface, smaller pore size, and larger pore volume compared to the PT prepared supports. The same trend for surface area, pore volume, and pore size was observed for the mixed oxide supports prepared by the PT and SA methods, and the same line of reasoning used earlier for explaining surface area reduction due to nickel incorporation and post-reforming reaction analysis for single oxide supports could be applied for mixed oxide supports. Generally, the incorporation of a second oxide resulted in a decrease in the specific surface area and pore volume of the single oxide support. This could be attributed to some level of introduction of one oxide into the pore structure of the other. On the contrary, La0.6Zr0.4 mixed oxide support prepared by the SA method exhibited a surface area larger than that of the constituent single oxides. This could indicate the complementary effect of adding La to Zr and that La contributes to the measured surface area of Zr. Table 3a shows that mixed oxides maintained a larger portion of the original surface area after the reforming reaction if compared with single oxides for supports prepared by PT. (54) Cheekatamarla, P. K.; Finnerty, C. M. J. Power Sources 2006, 160, 490–499.


Ibrahim and Idem

Figure 2. (a) XRD patterns of fresh calcined Ni catalysts (solid line) and supports (dotted line) for mixed oxide supports prepared by the PT and SA methods ((3) NiO, (4) ZrO2, (]) γ-Al2O3, (*) θ-Al2O3, (X) R-Al2O3, (0) CeO2, (O) La(OH)3, and (+) La2O3). (b) XRD patterns of fresh calcined Ni catalysts (solid line) and supports (dotted line) for mixed oxide supports prepared by the PT and SA methods ((O) γ-Al2O3, (×) LaAlO3, and (+) La2O3.

This was particularly evident with the La0.6Al0.4 mixed oxide support which maintained 73% and 64% of its original area after nickel impregnation and reforming reaction, respectively. This could be attributed to the improved thermal resistance of the surface area of La-modified alumina which is known to inhibit thermal sintering.55 Béguin et al.56 suggest that the mechanism by which this occurs is most likely related to the formation of microdomains of LaAlO3 on the Al2O3 surface, which will lock highly reactive coordinative unsaturated Al ions into a thermally stable structure. On the other hand, for mixed oxide supports prepared by the SA method, Ce0.6Zr0.4 exhibited very good surface features and maintained a high percentage of the original surface area both after nickel introduction (91%) and after the reforming reaction (63%). The results shown in Table 3a,b show that the specific surface area and pore volume for mixed oxide supports increased in the following order: La0.6Al0.4(SA) < Ce0.6La0.4(PT) < Ce0.6Zr0.4(PT) < La0.6Zr0.4(PT) < Ce0.6La0.4(SA) < La0.6Al0.4(PT) < Ce0.6Zr0.4(SA) < La0.6Zr0.4(SA). The results from Table 3a,b for mixed oxide supports indicate that the SA method of preparation provided a large specific surface, smaller pore size, and larger pore volume compared to the PT prepared supports, with La0.6Al0.4 prepared by the SA method being the only exception. 3.1.3. H2 Chemisorption (Dispersion) and Crystallite Size Estimation. The results for Ni metal dispersion, Ni surface area per gram of sample, and the surface-weighted crystallite sizes (55) Colussi, S.; de Leitenburg, C.; Dolcetti, G.; Trovarelli, A. J. Alloys Compd. 2004, 374, 387–392. (56) Béguin, B.; Garbowski, E.; Primet, M. Appl. Catal. 1991, 75, 119– 132.

are summarized in Table 4. Catalysts prepared by the SA method exhibited higher metal dispersion compared to their counterparts prepared by the PT method. Ni/Al2O3 prepared by PT exhibited a high level of metal dispersion value of 12.6%. According to our earlier work57 the high level of nickel dispersion on the surface of alumina could be attributed to the high surface area available for deposition and possibly the strong metal–support interaction which results in well dispersed metal probably in the form of nickel aluminate spinel (NiAl2O4). The nickel surface area per gram of sample showed a similar trend to the metal dispersion results. Generally, there was no evident improvement in metal dispersion upon the incorporation of single oxides to mixed oxide support. On the other hand, the mixed oxide La0.6Al0.4 system enhanced the metal dispersion of the La2O3 single support by incorporating it with the high surface area alumina. The metal dispersion for single oxide support catalysts increased in the following order: La2O3(PT) > ZrO2(PT) ) La2O3(SA) > CeO2(PT) > ZrO2(SA) > CeO2(SA) > Al2O3(PT). The crystallite size measurements for single oxide supports follows a trend opposite to that for the metal dispersion values, as expected. CeO2-supported catalysts prepared by the SA method exhibited a relatively lower crystallite size compared to the La2O3 and ZrO2 systems. Ceria-based catalysts, regardless of the preparation method, exhibited a relatively higher metal dispersion and smaller crystallite size as compared to the other rare earth-containing supports. There was no significant effect on metal dispersion upon combining single oxides to form mixed oxides, and the values



Figure 3. (a) TPR profiles of fresh calcined Ni catalysts (solid line) and supports (dotted line) for pure oxide supports prepared by the PT and SA methods. (b) TPR profiles of fresh calcined Ni catalysts (solid line) and supports (dashed line) for mixed oxide supports prepared by the PT and SA methods. Table 4. H2 Chemisorption (Ni Dispersion) and Crystallite Size Estimation Ni dispersion (%) Al2O3 CeO2 La2O3 ZrO2 Ce0.6Zr0.4 Ce0.6La0.4 La0.6Zr0.4 La0.6Al0.4 a

Ni surface area (m2 (g of sample)-1)

surface-weighted crystallite sizea (nm)

PT

SA

PT

SA

PT

SA

12.6 2.3 1.3 1.8 2.0 1.4 1.6 1.9

npb 3.3 1.8 2.6 2.2 1.8 1.7 8.1

2.5 0.7 0.4 0.5 0.6 0.7 0.4 0.7

npb 1.0 0.5 0.7 0.8 0.9 0.6 1.6

8.0 43.5 80.8 57.7 45.9 72.1 62.3 54.0

npb 30.5 56.7 41.1 45.9 55.5 58.4 12.4

Calculated using the formula ds (nm) ) 101/D (%).66 b np: not prepared.

remained within the margin of error. It appears from the results that the combination of alumina and rare earth significantly improves the dispersion of the latter. The Ni surface area per gram of sample shows the same trend as Ni dispersion while the crystallite size follows the inverse trend. The metal dispersion for mixed oxide-supported catalysts increased in the following order Ce0.6La0.4(PT) > La0.6Zr0.4(PT) > La0.6Zr0.4(SA) > Ce0.6La0.4(SA) > La0.6Al0.4(PT) > Ce0.6Zr0.4(PT) > Ce0.6Zr0.4(SA) > La0.6Al0.4(SA). 3.1.4. XRD. Powder XRD patterns of the fresh calcined supports (i.e., single and mixed oxides) and catalysts prepared by PT and SA are presented in Figure 2a,b . The single Al2O3 system prepared by PT exhibited reflections at 2θ angles of 24°, 36.5°, 43.5°, and 67.4°. These peaks confirm the phase transformation due to thermal treatment of the γ-Al2O3supported catalysts to R and θ phases.58,59 This result confirms the earlier observation of surface area loss due to the alumina (57) Ibrahim, H. H.; Kumar, P.; Idem, R. O. Energy Fuels 2007, 21, 570–580.

phase transition. The supports containing cerium, for both preparation methods, show strong reflections at 2θ values of 28.4°, 32.9°, 47.3°, 56.2°, and 70° which represent the indices of (111), (200), (220), (311), and (400) planes of CeO2, respectively. This indicates the presence of a cubic fluorite structure for ceria containing supports. However, the peaks were broader and had lower intensity in the case of Ni/CeO2 prepared by the PT method, which is indicative of smaller crystallites or poor crystalline state.55 No evidence of sintering or phase transformations due to calcination of catalysts was observed for Ni/CeO2 and Ni/Ce0.6Zr0.4 catalysts prepared by the SA method, which indicates the effectiveness of the SA method in producing a more stable carbon resistant catalyst. Typical diffraction peaks of NiO crystalline phase usually obtained at 37.3°, 43.2°, and 62.9° were not detected by XRD for the calcined catalysts. This (58) Schaper, H.; Doesburg, E. B. M.; van Reijen, L. L. Appl. Catal. 1983, 7, 211–220. (59) Lippens, B. C.; de Boer, J. H. Acta Crystallogr. 1964, 17, 1312– 1321.


Ibrahim and Idem

Table 5. Summary of TPR Analysis for the Fresh Calcined Catalysts number of peaks Al2O3 CeO2 La2O3 ZrO2 Ce0.6Zr0.4 Ce0.6La0.4 La0.6Zr0.4 La0.6Al0.4

PT

SA

2 3 3 2 2 2 2 2

npb 5 2 3 4 2 2 4

peak temperature (°C) PT 560, 330, 340, 563, 418, 285, 450, 860,

860 406, 975 505, 759 870 665 440, 664, 740 618 930

SA npb 260, 480, 320, 460, 439, 490, 450,

383, 744 610, 680, 703 710 590,

505, 710, 883 720 800, 920 840, 980

major reducible species PT NiAl2O4, NiO Ce4+, NiO La3+, NiO NiO Ce4+, NiO Ce4+, La3+, NiO La3+,NiO NiAl2O4, LaAlO3, NiO

reducibilitya (%)

SA

PT

SA

Ce4+, NiO La3+, NiO NiO Ce4+, NiO Ce4+, NiO La3+, NiO NiAl2O4, LaAlO3, NiO

15 51 45 34 45 45 30 50

npb 58 47 44 45 43 29 31

npb

Reducibility was measured via the convolution theory and was defined as the fraction of area under the curve reduced at the temperature of 700 °C with respect to the entire area under the TPR curve. b np: not prepared. a

indicates that nickel loading (5 wt %) on the catalysts is within the monolayer coverage, that NiO is amorphous, or that the NiO particle size is too small (i.e., beyond the detection limits) to be observed at lower loadings. This phenomenon is consistent with low nickel loadings as reported by Kim et al.60 and Guo et al.61 Diffraction patterns for Al2O3 containing catalysts prepared by the PT method and for CeO2 containing catalysts prepared by the SA method exhibited relatively broader peaks corresponding to Al2O3 and CeO2 crystalline phases, indicating smaller crystallite sizes. This is in total agreement with our earlier findings obtained with H2 chemisorption analysis for active metal dispersion. Also, the same reflections observed on the Ni/CeO2 system were observed on the Ni/Ce0.6Zr0.4O2 system suggesting the presence of a true mixed-oxide phases with cubic fluorite structure. It should be noted that the diffraction peaks shifted to higher degrees when the Ni/Ce0.6Zr0.4O2 catalyst was prepared by the SA method. This could be attributed to the shrinkage of the lattice because of the replacement of Ce4+ (0.097 nm) with a smaller cation Zr4+ radius (0.086 nm), which indicates the superiority of the SA method in producing a true solid solution with respect to the PT method. There is no indication of the presence of other phases such as ZrO2 or CeO2. This also is indicative that Ce and Zr ions are homogeneously mixed. Figure 3b shows the presence of the thermally stable structure LaAlO3 on the La-modified alumina prepared by the SA method, which confirms our earlier observation on surface area measurements. 3.1.5. Temperature Programmed Reduction. Figure 3a,b shows the H2-TPR profiles of single and mixed oxide supports and catalysts prepared by the PT and SA methods. Also, the number of peaks observed, peak temperature, major reducible species, and the percent reducibility of catalysts are summarized in Table 5. Also, Figure 3a,b shows that, for all the supports prepared by the PT and SA methods, the incorporation of Ni results in a shift of the TPR profile toward lower reduction temperatures with respect to that of single support. Teschner et al.62 attribute this phenomenon to the presence of metal which strongly modifies the support features as a result of hydrogen activation by the metal and consequent migration to the support (spillover) favoring reduction of the ceria surface at a lower temperature. The Ni/Al2O3 prepared by the PT catalyst showed the least reducibility (15%, Table 5) with two peaks corresponding to the reduction of Ni species that are not completely integrated in the spinel structure and to the presence of NiAl2O4 spinel at 560 and 860, respectively.53 For samples with low nickel loadings such as the ones used in this study, the formation (60) Kim, P.; Kim, Y.; Kim, H.; Song, I. K.; Yi, J. Appl. Catal., A 2004, 272, 157–166. (61) Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Appl. Catal., A 2004, 273, 75–82. (62) Teschner, D.; Wootsch, A.; Ro¨der, T.; Matusek, K.; Paa’, Z. Solid State Ionics 2001, 141, 709–713.

of Ni-Al2O4 spinel phase has been previously reported.63 It is typically a reduction peak at relatively high temperatures (>800 °C) corresponding to the reduction of a thermally stable phase such as the NiAl2O4 structure present because of the strong interaction between the highly dispersed NiO and the Al2O3 support. Hence, increasing the reducibility of the Ni species (such as in nickel catalysts promoted with cerium and lanthanum) reduces carbon formation in POX of methane reactions.64,65 Also, XRD analysis confirms the formation of the NiAl2O4 as can be seen in Figure 2a. Two peaks are observed at 44° and 65° which could be attributed to the spinel phase NiAl2O4 possibly as a result of the NiO-support interaction with the relatively high calcination temperature used in this study. This corroborates the work of Christensen et al.66 on the effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming. This observation is in total agreement with the Ni dispersion results obtained earlier where the same catalyst exhibited a high level of metal dispersion. The single CeO2 support prepared by the SA method exhibited reduction peaks at 530, 650, and 900 °C. The two lower temperature reduction peaks are assigned to the reduction of dispersed ceria, whereas the peak at 900 °C is assigned to the reduction of bulk CeO2.67 The Ni/CeO2 catalyst prepared by the SA method had the highest reducibility (58%) with five distinguishable peaks at 260, 383, 505, 710, and 883 °C, characteristic of reduction of NiO particles and noncrystalline and surface dispersed NiO species (700 °C).68 The PT prepared CeO2 support showed typical two peaks for ceria at 580 °C as a result of the reduction of the most easily reducible surface-capping oxygen of ceria followed by a second peak at 940 °C as a result of the removal of bulk oxygen. The Ni/CeO2 catalyst prepared by the PT method did not show the same level of reducibility with only three peaks, that is, at 330, 406, and 975 °C, only two of which are within the range of the reduction temperature used for this study. This indicates that the catalyst preparation method (SA) enhances greatly the degree of catalyst reducibility. This could be ascribed to the generation of oxygen vacancies facilitated by the SA method, which adsorb oxygen easily. Therefore, very reactive oxygen species are (63) Basile, F.; Basini, L.; D’ Amore, M.; Fornasari, G.; Guarinoni, A.; Matteuzzi, D.; Del Piero, G.; Trifiro, F.; Vaccari, A. J. Catal. 1998, 173, 247–256. (64) Lucrédio, A. F.; Jerkiewickz, G.; Assaf, E. M. Appl. Catal., A 2007, 333 (1), 90–95. (65) Guo, J.; Lou, H.; Zhao, H.; Chai, D.; Zheng, X. Appl. Catal., A 2004, 273 (1-2), 75–82. (66) Christensen, K. O.; Chen, D.; Lødeng, R.; Holmen, A. Appl. Catal., A 2006, 314 (1), 9–22. (67) Yao, H. C.; Yao, Y. F. J. Catal. 1984, 86, 254–265. (68) de Leitenburg, C.; Trovarelli, A.; Kaspar, J. J. Catal. 1997, 166, 98–107.



formed, which are reduced easily by H2 at low temperature.69 Single ZrO2 supports did not show any reduction peaks for PT and SA whereas La2O3 supports exhibited two reduction peaks at 558 and 740 °C for PT and 580 and 840 °C for SA. Also, the mixed oxide supports containing La did not show any reduction peak associated with La species. For mixed oxide supports, Ce0.6Zr0.4O2 showed the highest reducibility (45%) for catalysts prepared by PT and SA. Ni/Ce0.6Zr0.4O2 catalyst prepared by the PT method had two reduction peaks at 418 and 665 °C attributed to the reduction of Ni species and highly dispersed CeO2 in Ce0.6Zr0.4O2 solid solution, respectively. The same catalyst prepared by the SA method showed a similar trend with the addition of two new broad peaks at 800 and 920 °C. The broader peaks with SA are indicative of a stronger metal–support interaction, which is believed necessary to offer a stable activity during POX of hydrocarbons. 3.2. Catalytic Activity Tests. The performance for iso-octane and gasoline POX reforming on the single and mixed oxidesupported nickel is presented in terms of conversion (X), hydrogen selectivity (S), and hydrogen yield (Y) defined respectively as given in eqs 5-7, as well as in terms of activity (X, S, and Y) stability with TOS. In the case of X, the absence of significant concentrations of C2+-containing products allowed the calculations on the basis of only C1 components, that is, CO, CO2, and CH4.

Figure 4. Hydrogen yield vs TOS during iso-octane reforming on catalysts prepared by the PT method, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5.

Conversion (iso-octane/gasoline) % ) (CO)out(CO2)out(CH4)out × 100 (5) 8(C*)in where C* refers to the moles of iso-octane or gasoline fed into the system. Selectivity (H2) % ) Yield (H2) % )

(H2)out (H2)theoretically expected × X

× 100

(H2)out × 100 (H2)theoretically expected

(6) (7)

All the catalysts prepared by the PT and SA methods were first screened for their activity and their stable performance for H2 production by POX of iso-octane at 700 °C and an O2/C ratio of 0.5. The best catalysts were further tested for stability and then for H2 yield for H2 production by POX of sulfur-free gasoline. The best overall catalyst with the most stable performance was then used to study the dependencies of product distribution and H2 yield on the operating conditions (i.e., reforming temperature, O2/C ratio, and gas hourly space velocity) during the CPOX reforming of gasoline. Also, a longterm stability study of the catalyst was investigated for the POX gasoline reforming. 3.2.1. Iso-Octane System. The catalytic H2 yield for single and mixed oxide-supported Ni catalysts prepared by the PT method as a function of TOS is given in Figure 4. It can be seen that although most of the catalysts start very high, the H2 yield drops very rapidly especially within the first 4 h TOS. It appears that most of the deactivation seen with catalysts prepared by the PT method is due to coke formation and active surface area loss as was evidenced from the BET analysis given in Table 3. Deactivation of catalysts because of catalyst sintering is another unfavored possibility especially for the La-modified supported Ni catalysts in the light of our earlier discussion that La enhances the thermal stability and extends the window of (69) Shan, W.; Luo, M.; Ying, P.; Shen, W.; Li, C. Appl. Catal., A 2003, 246, 1–9.

Figure 5. Hydrogen yield vs TOS during iso-octane reforming on catalysts prepared by the SA method, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5.

operation toward higher temperatures. The figure also shows that 5Ni/CeO2 catalyst achieved the highest H2 yield. However, the stability was not as good and the catalyst stability kept on dropping with TOS. Figure 5 shows the catalytic H2 yield for single and mixed oxide-supported Ni catalysts prepared by the SA method as a function of TOS. In general, all the catalysts exhibited a more stable performance as compared to the corresponding catalysts prepared by the PT method especially after 120 min TOS. Two catalysts in particular, namely, Ni/CeO2 and Ni/La2O3, exhibited an exceptional level of stability toward hydrogen production with the former producing 10% more H2. Also, other catalysts that showed promising stability were Ni/La0.6Zr0.4 and Ni/ Ce0.6Zr0.4. The same line of reasoning used previously could also be used to explain the higher degree of resistance to deactivation exhibited by this group of catalysts. The high level of stability associated with the SA method of support preparation could be ascribed to their higher surface areas, better thermal stability, good level of metal dispersion, and higher reducibility. These results show that the SA method is effective to prepare more stable catalysts than the PT method. 3.2.2. Gasoline System. Four catalysts prepared by the SA method, namely, 5Ni/CeO2, 5Ni/La2O3, 5Ni/La0.6Zr0.4, and 5Ni/ Ce0.6Zr0.4, and one catalyst prepared by the PT method, namely, 5Ni/La0.6Zr0.4, were short listed for the study on POX gasoline on the basis of their catalytic stable activities derived from earlier tests. The H2 yield and gasoline conversion for these catalysts are presented in Figure 6a,b, respectively, as a function of TOS. Generally, it could be seen that the catalysts did not suffer the


Ibrahim and Idem Table 6. Typical Calculated Thermodynamic Equilibrium and Experimentally Measured Product Composition for the 5% Ni/CeO2 Catalyst Prepared by the SA Method

Figure 6. (a) Hydrogen yield vs TOS during synthetic gasoline reforming on catalysts prepared by the PT and SA methods, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5. (b) Conversion vs TOS during synthetic gasoline reforming on catalysts prepared by the PT and SA methods, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5.

same level of drop in H2 yield seen in Figures 4 and 5 for the entire TOS range. This could be ascribed to the fact that the reactivity of the synthetic gasoline mixture is not simply an average of the individual components, which explains the trend seen in Figure 6.70 We are still studying this phenomenon so as to derive a better understanding of the effects of individual gasoline components on catalysts’ reformability. Figure 6b shows that for the 5Ni/Ce catalyst prepared by the SA method the conversion values remained higher and more stable than all the catalysts tested for the synthetic gasoline reforming. A blank (noncatalytic) run was also performed to serve as a benchmark and show the extent of contribution of the thermal cracking. An H2 yield of about 20 mol % was found to be due to merely thermal cracking. Out of the catalysts tested, 5Ni/CeO2 was the best overall catalyst. None of the other tested catalysts had a similar stability as shown by the single ceria-supported nickel catalyst. The 5Ni/La0.6Zr0.4 prepared by the PT method suffered from rapid H2 yield decline and poor stability. On the other hand, the 5Ni/La0.6Zr0.4 prepared by the SA method had a much better stability as compared to the corresponding PT prepared catalyst. This shows that the same support prepared by two different methods can result in two different trends. The 5Ni/ Ce0.6Zr0.4 also presented a good level of stability except for the last portion of the reaction where the H2 yield dropped slightly. The excellent performance of 5Ni/CeO2 in terms of stability for the POX of gasoline can be attributed to its higher reducibility. These characteristics exhibited by 5Ni/CeO2 enables (70) Subramaniana, R.; Panuccioa, G. J.; Krummenachera, J. J.; Leeb, I. C.; Schmidt, L. D. Chem. Eng. Sci. 2004, 59, 5501–5507.

component

chemical symbol

experimentally measured composition

equilibrium calculated composition

hydrogen carbon monoxide carbon dioxide methane

H2 CO CO2 CH4

44.8 34.6 15.3 5.3

52.7 33.0 9.9 4.4

the support to make good use of its OSC and to participate in the redox function of the catalyst, thus increasing its stability during the POX reforming of gasoline. The main purpose of this study is to develop stable catalysts for the CPOX of gasoline. The Ni/CeO2 catalysts and Ni/Ce0.6Zr0.4 prepared by the SA method exhibited the best stable activity and H2 selectivity as can be seen in Figure 6. A typical thermodynamic equilibrium product composition calculation performed based on the literature71 for this system at 700 °C is presented in Table 6. The product composition is based on a N2-free and dry basis. The calculations for equilibrium conversion at the given conditions showed a higher degree of conversion (78%) compared to the actual average experimental conversion (72%). This explains the expected deviation from equilibrium of the experimental exit composition values. 3.3. Catalyst Characteristics and Activity Correlations. Results obtained for the catalytic activity (Figures 4-6) of single and mixed oxide-supported catalysts prepared by the PT and SA methods of preparation clearly reveal the superiority of the SA method of preparation in producing catalysts with better structural features which leads to a more active and stable hydrogen production. The better performance of the Ni/CeO2 catalyst prepared by the SA method toward the stable hydrogen production during the POX reforming of gasoline is attributed to its high thermal stability, high surface area, better metal dispersion, and high reducibility. It is believed that the catalyst ability to resist excessive carbon formation on its surface via its high oxygen mobility and formation of smaller nickel crystallite sizes which upset the point of carbon nucleation is the main reason for its good performance.6,72 The ability of the cubic Ni/CeO2 is related to its rapid reduction/oxidation capability by releasing and up-taking oxygen owing to the reversible reaction of CeO2.27,44 Kumar et al.32 reported recently on the high catalytic activity and stability of nickel-based catalysts prepared by SA during the dry reforming of methane. We have provided a few examples (Figures 7-9) to show possible correlations between catalyst characteristics and catalyst activity. Figure 7 shows the effect of metal dispersion on the catalytic activity and selectivity during the POX of gasoline. Even with the typical measurement errors in the activities, the results show that there appears to be a direct relationship between metal dispersion and gasoline conversion, H2 selectivity, and H2 yield. This is believed to be a direct result of the catalyst ability to resist or minimize catalyst deactivation via thermal and/or coke formation resistance. High metal dispersion is usually accompanied by a smaller metal crystallite size as shown in Table 4. The use of the SA method of support preparation is believed to result in a smaller metal ensemble28 which in turn prevents nickel particle agglomeration, the main (71) Pe˜na, M. A.; Gómez, J. P.; Fierro, J. L. G. Appl. Catal., A 1996, 144, 7–57. (72) Rostrup-Nielsen, J. R. Stud. Surf. Sci. Catal. 1991, 68.



Figure 7. Effect of short listed catalyst metal dispersion on gasoline conversion, H2 selectivity, and H2 yield. Figure 10. Hydrogen yield and production composition as a function of reforming temperature during POX of gasoline, GHSV ) 7 802 800 h-1, and O2/SG ) 4.

Figure 8. Effect of short listed catalyst reducibility on gasoline conversion, H2 selectivity, and H2 yield.

Figure 9. Effect of short listed catalyst surface area reduction after reaction on gasoline conversion, H2 selectivity, and H2 yield.

reason behind coke population growth.28,73 The effect of catalyst reducibility on the gasoline conversion, H2 selectivity, and H2 yield is depicted on Figure 8. The general trend for all three activity measures (X, S, Y) shows a direct relationship between reducibility and hydrogen production. The catalyst reducibility is believed to provide a good measure of its intrinsic catalytic activity. It is not enough to have a well dispersed metal on the support surface; rather, how much of it could be activated via reduction to its metallic state and be available for reaction is also important. Another characteristic feature that correlates well with the catalytic performance is the percent of area reduction after reaction, given in Figure 9. The figure shows an increase in X, S, and Y with the catalyst surface area after reaction. The surface area maintained after reaction provides us with insight on the textural stability of catalysts after several hours of TOS. This gives an indication of the degree of thermal and mechanical (73) Rostrup-Nielsen, J. R. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M. Eds.; Springer-Verlag: Berlin, 1984; Vol. 5.

stability of the catalyst during the POX reforming of gasoline. A high catalyst surface area after reaction would imply that more active metal surface is exposed and available during reforming reaction. This indicates mechanical and thermal stability which prevents structural collapse and maintains a relatively more intact structure. Moreover, a high surface area after reaction is indicative of a high level of resistance to sintering and active site coverage because of coking and/or fouling. 3.4. Effect of Reforming Temperature. The effect of reforming temperature during POX of gasoline over 5Ni/CeO2 catalyst on the H2 yield and product composition is given in Figure 10. It can be seen that there is a direct relation between reforming temperature and H2 yield with a sharp rise at temperatures above 600 °C. Also, a rise in the reaction temperature resulted in an increase in the product concentrations with the exception of CO2, which had a sharp fall at temperatures higher than 600 °C. CH4 did not exist at temperatures less than or equal to 600 °C and started to form above 600 °C. The sudden increase in CH4 and simultaneous drop in CO2 concentrations could be attributed to the fact that methanation reaction is favored at high temperatures. Thus, part of the H2 was consumed to react with CO2 and produce CH4. Furthermore, reverse water gas shift reaction will definitely take place at elevated temperatures. This was reflected in the high CO/H2 ratio in the product distribution. This explains why the H2 concentration had a consistent level of increase with temperature even at temperatures above 600 °C. Also, the absence of CH4 in the product stream could imply that adsorbed organic molecules react with oxygen on the catalyst surface until all carbon atoms are converted mainly to CO or CO2.1 This confirms the importance of the OSC feature of the CeO2supported Ni system. 3.5. Effect of O2/SG Ratio. The product distribution and H2 yield as a function of oxygen to synthetic gasoline (O2/SC) ratio for the reforming of synthetic gasoline at 600 °C is given in Figure 11. The figure shows that H2 yield increases with O2/ SC to a maximum of 7 mol % (N2 balanced) at 600 °C. CH4 was not detected for the entire range of the O2/SC ratio used in this study. The concentration of CO increased from 1.9 to 5.9% with an increase in the O2/SC ratio from 1 to 4 while the CO2 concentration almost remained unchanged. The high levels of CO2 could be ascribed to solid carbon gasification or to CO conversion to CO2. 3.6. Effect of the Hourly Space Velocity. The product distribution and H2 yield as a function of GHSV for the reforming of synthetic gasoline at 600 °C and O2/SG ratio of 4


Ibrahim and Idem

Figure 11. Effect of feed ratio on the hydrogen yield and product distribution during gasoline reforming, T ) 873 K, and GHSV ) 7 802 800 h-1.

Figure 13. (a) Conversion stability test for the 5NiCe catalyst prepared by the SA method during synthetic gasoline reforming, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5. (b). H2 selectivity stability test for the 5NiCe catalyst prepared by the SA method during synthetic gasoline reforming, T ) 700 °C, GHSV ) 940 000 h-1, and O2/C ) 0.5. Figure 12. Hydrogen yield and production composition as a function of GHSV during POX of gasoline, T ) 873 K, and O2/SG ) 4.

is shown Figure 12. It can be seen that the H2 yield dropped from ∼20 to 3% for an increase in the GHSV from 1.5 × 105 to 2.3 × 106 h-1. The results also show that CH4 remained unseen for the range of conditions used. The H2 concentration in the effluent gas dropped from 11 to 5% associated with a simultaneous drop in CO from 10 to 4%. The increase in CO2 concentration is probably due to the increase in O2 conversion via hydrocarbon total oxidation. 3.7. Catalyst Stability Test. The catalytic stability obtained over 5Ni/CeO2 catalyst is presented in Figure 13a,b, which shows the variation in conversion and H2 selectivity as a function of TOS. The stability experiment was conducted at 700 °C with a sulfur free synthetic gasoline mixture. The catalyst was kept for 8 h on stream during which time the catalyst exhibited a very stable performance and there was no sign of deactivation after 3 h TOS. This represents a very encouraging result in the sense that such catalyst stability for reforming liquid hydrocarbons without dilutions, to our knowledge, has not been reported in the open literature. Results obtained from TPO analysis and BET measurements in our earlier work57 have shown that the source of catalyst deactivation is mainly due to a combination of sintering and coke formation and not just one simple mechanism. Results presented in Table 3b of this work for catalysts prepared by the SA method show that the catalysts maintained a larger portion of the original surface area when analyzed after reaction as can be seen from Table 3b. This is a strong indication of a

lower level of catalyst sintering. Furthermore, no carbon flakes or deposits were observed on the interior surface of the reactor wall. In the case of the catalyst with the most stable performance (i.e., 5Ni/Ce prepared by SA), there was no sign of catalyst deactivation during the TOS as can be seen from Figure 13a,b. This was attributed to the lower degree of surface area loss after reaction (Table 3b) and the high OSC known for Ce supports. The OSC of the catalyst plus the O2 available from the POX reforming is believed to provide the ability to gasify deposited carbon at a rate faster than the rate at which they are formed. 4. Conclusions The CPOX reforming of iso-octane and sulfur-free synthetic gasoline mixture over single and mixed oxide-supported Ni catalysts prepared by two techniques, namely, the PT and SA methods, has been investigated with respect to the structural and morphological characteristics of the dispersed metallic phase and the supports. The results indicate that the SA preparation of the support provides a mesoporous, thermally stable material that, in combination with appropriate metal loadings of Ni, leads to a very stable catalyst for CPOX of sulfur free synthetic gasoline. It appears that the presence of surfactant induces tension reduction during the drying and calcination processes resulting in a better material as compared to the PT method. The high level of stability associated with the SA method of support preparation was due to their higher surface areas, better thermal stability, good level of metal dispersion, and degree of reducibility. Thus, the SA method is effective to prepare more stable catalysts than the PT method. The 5Ni/CeO2 catalyst prepared by the SA method exhibited high stability via


improving thermal resistance and lowering coke formation by the OSC feature during the CPOX reforming of synthetic gasoline. The SA catalysts also yielded excellent performance results when compared with other catalysts for the POX reforming of gasoline and iso-octane. Furthermore, the SA catalysts developed are also more stable than any catalyst


previously used or reported for the POX reforming of gasoline. Acknowledgment. The authors would like to thank the Auto 21 Network of Centers of Excellence for their financial support. EF7005904

Single and Mixed Oxide-Supported Nickel Catalysts for the Catalytic

Recommend Documents